Ventilation And Perfusion

Question 1

How does the pulmonary circulation run in relation to the systemic circulation ?

Answer 1

Pulmonary circulation runs parallel with the systemic circulation

Question 2

What is systemic circulation ?

Answer 2

Flow of blood from the left ventricle to the body and then to the right atrium
- not every organ receives all of the blood from the systemic circulation

Question 3

What is the gas composition of the blood in the systemic circulation ?

Answer 3

• oxygen rich
• carbon dioxide poor

Question 4

What is the gas composition of the blood in the pulmonary circulation ?

Answer 4

• oxygen poor
• carbon dioxide rich

Question 5

What is the arterial blood pressure and pressure difference in the pulmonary circulation ?

Answer 5

• blood pressure = 25/10 mmHg
• pressure difference = 13 mmHg

Question 6

What is the arterial blood pressure and pressure difference in the systemic circulation ?

Answer 6

• blood pressure = 120/80 mmHg
• pressure difference = 95 mmHg

Question 7

Describe the structure and distribution of pressure in pulmonary vessels

Answer 7

• pulmonary arterial walls are thin and contain little smooth muscle
• pulmonary vascular resistance is low
• distribution of pressure in pulmonary vessels is relatively symmetrical across the circulation

Question 8

What is vascular resistance ?

Answer 8

Vascular resistance =
(input pressure - output pressure) / blood flow

Question 9

What is perfusion ?

Answer 9

The flow of blood to the lungs

Question 10

What is the ventilation : perfusion ratio ?

Answer 10

It gives an index of the efficiency of the lung for gas exchange

Question 11

What is the average ventilation : perfusion ratio in the lungs ?

Answer 11

Alveolar ventilation of the whole lung = 4.5 litres/min
Cardiac output = 5.5 litres/min
So average ventilation : perfusion = 0.8
(ratio needs to be approx 1 for good gas exchange)

Note - local ratio can vary across the lung

Question 12

How does gravity influence ventilation and perfusion ?

Answer 12

In the upright standing position, both perfusion and ventilation increase from the apex of the lung to the base so gravity increases both ventilation and perfusion
- there is good matching of ventilation and perfusion

Question 13

Describe ventilation in an upright individual

Answer 13

• interpleural pressure is greater (more negative) at the apex of the lungs
• distending forces are greater at the apex
• and so alveolar volume is greater at the apex and so there is less ventilation

Question 14

Describe perfusion in an upright individual

Answer 14

• At the top of the lung -
alveolar pressure > arterial pressure > venous pressure
• At the base of the lung -
arterial pressure > venous pressure > alveolar pressure
• blood pressure is greater at the base of the lung

Question 15

Which other factors have an affect on distribution …

Answer 15

• posture
• exercise
• disease

Question 16

Why are there local/regional variations in the ventilation : perfusion ratio ?

Answer 16

Local blood flow falls ~ 3 times faster than ventilation and so regional ventilation : perfusion ratio changes across the lung

Question 17

What can regional ventilation : perfusion vary from ?

Answer 17

• it can vary from 0 to infinity

0 = blood passing through the lung without coming into contact with alveolar air (0/perfusion)
Infinity = anatomical dead space or ventilated alveoli that are not perfused (ventilation/0)

Question 18

How does ventilation : perfusion ratio influence blood gas composition ?

Question 19

How does ventilation : perfusion mismatching affect blood gases ?

Answer 19

• well ventilated and perfumed alveoli = ratio of 1
• poorly ventilated but well perfused alveoli = ratio less than 1
• well ventilated but poorly perfused alveoli = ratio of greater than 1

Question 20

What are the mechanisms used to defend ventilation : perfusion matching ?

Answer 20

• principally achieved by modulation of blood flow rather than ventilation
• vasoconstriction of capillaries by hypoxia
• blood is directed away from poorly ventilated areas
• response is very non linear

Question 21

Where does gas exchange occur ?

Answer 21

Gas exchange occurs across the alveolar - capillary membrane
- they move by simple diffusion from a region of high partial pressure to low partial pressure

Question 22

Where is carbon dioxide and oxygen carried in the blood ?

Answer 22

• carbon dioxide is carried predominantly in the chemical combination of plasma
• oxygen is bound with haemoglobin in erythrocytes

Question 23

What influences the transfer of oxygen ?

Answer 23

• diffusion across the red blood cell membrane
• combination with haemoglobin